Compliant hydrodynamic gas bearings are subjected to two principal types of instability, the first of which is known as "synchronous whirl" and the second of which is known as "half-speed whirl". During relatively low-speed rotation of a shaft, the orbiting motion of the geometric center of the shaft about the geometric center of the bearing support tends to set up centrifugal forces acting on the shaft which cause the shaft to orbit or whirl at a rotational speed equal to the rotational speed of the shaft about its own axis. This orbiting or whirling motion is synchronous whirl and occurs at the lowest critical speed of the bearing.
Half-speed whirl is a more serious instability which occurs as the shaft approaches a speed approximately equal to twice its lowest critical speed. At twice critical speed the shaft inherently tends to undergo harmonic vibration at its lowest critical frequency. This harmonic vibration is superimposed upon the synchronous whirl and is stimulated or excited by the load-carrying rotating fluid-wedge whose average velocity about the shaft now approaches the lowest critical speed. As a result, orbital excursions of the shaft rapidly increase in amplitude. During half-speed whirl the whirl velocity of the shaft approximates the average velocity of the fluid-wedge. When this occurs the speed of the fluid-wedge relative to the orbiting shaft tends toward zero, causing a loss of fluid-film support. Since the shaft is operating at a relatively high speed, contact between the shaft and bearing may cause damage or failure of the bearing.
The aforesaid problem has heretofore been addressed by selection of a spring rate for the foil support system that minimizes the problem by altering the shape of the fluid-film to maintain stability and location of the mounted shaft group. No provision was made for controlled adjustment of shaft location and/or adjustment of foil spring rate.